Load Charts Calculations for Offshore Pedestal Cranes by Motion Response Analysis
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- Institutt for marin teknikk 
In this Master's thesis the sea lift operation from a supply vessel onto a oil platform has been evaluated. Two models have been built and analysed with different input parameters, as well as compared with the current load charts based on the calculation procedure found in the standard EN 13852-1.The most critical part of the sea lift operation with regards to the structural safety of the crane is the lift-off from the supply vessel deck. The snap-load in the lift wire will depend on the motions of the lifted object, which is following the supply vessel deck motions. For floating platforms the motions of the crane tip must be accounted, and hence the coupled system of the supply vessel and the platform must be evaluated to estimate the extremal snap-loads in the lifting wire.The two simulation models have been developed in the simulation program SIMA, which is a state of the art simulation program developed especially for simulation of marine operations. The first model is built to represent a fixed platform, and is hence a general model representing bottom-supported and jack-up platforms. The second model is modelled as a semi-submersible platform, representing the Kristin platform located at Haltenbanken in the North-Sea. The wave statistics for Haltenbanken have been analysed to find the most common significant wave heights, spectral peak periods and wave headings for the area over time. The extremal snap-loads are found by fitting a Gumbel distribution to the respective simulation results sample.It is found that the probability level of exceedance for the dynamic factors given by EN-13852 are deviating between the models. For the different significant wave heights for the fixed platform, the EN 13852-1 represents a probability of exceedance of 0.999 when the container is placed in the stern end of the supply vessel deck. For the floating platform the same probability of exceedance are roughly 10\% below the standard, and is hence much more conservative.\\\\The effect of the spectral peak period has been evaluated by varying the spectral peak period for a constant wave height. It is found a close correlation between the pitch motion amplitude ratio and the extremal snap-load. For the fixed platform, the maximum snap-load occurs for a spectral peak period of 9 seconds: the same spectral peak period as for the maximum pitch motion amplitude ratio. For the floating platform the maximum snap-load occurs for a wider range between 9 and 11 seconds. A limitation of the increased lifting potential by adding the spectral peak period in the load charts, are the maximum snap-loads occuring for the most common spectral peak periods in the North-Sea.\\\\The difference between the different extremal snap-loads for the different load locations on the supply vessel deck are found to increase with an increasing pitch motion amplitude. Hence will the maximum lifting potential occur for the most common conditions at Haltenbanken. The difference for the floating platform between a midship location and a location in the stern end of the supply vessel deck is 26\% for a significant wave height of 3 meters.The effect of the wave heading is analysed for the floating platform. The lowest snap-loads are found for a 45 degrees wave heading in the range from 0 to 90 degrees. However, the deviations are only significant for the higher range of the significant wave heights.Based on the findings in the analyses, an offshore crane load-chart calculation procedure is proposed. It it suggested that the vertical motions of the supply vessel is estimated by several cameras, and that the corresponding extremal snap-loads to be expected for different load locations establishes the load-chart. This will be much more feasible and accurate than basing the load-charts on the motions of a worst-case supply vessel design.